Biota of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 175

Freshwater mussels are also one of the most imperiled groups of organisms in the world, and
about 70% of North American species is considered threatened, endangered, or extinct (Lydeard
et al. 2004; Strayer et al. 1999; Figure 6.13). One of the causes of mussel decline in the United
States was overharvesting of mussels for making buttons or for using shell fragments for seeds in
the cultured pearl industry. Habitat alterations such as due to dams or channelization for navigation
also contributed to the decline of the mussel populations in the United States, as did pollution and
excess sedimentation.
Another factor that resulted in the decline of mussels in some areas was an invasive species, the
zebra mussels. The zebra mussels, originally native to the Caspian Sea, were released into the Great
Lakes in 1988. Presumably, the zebra mussels were transported and released into ballast water.
According to the USGS Great Lakes Science Center, in less than 10 years following their original
release, the zebra mussels had spread to all of the Great Lakes, as well as the Hudson and Ohio
River basins, and the entire length of the Mississippi (Kirk et al. 2001). The zebra mussel distribu-
tion of May 2011 is indicated in Figure 6.14, from the U.S. Zebra Mussel Sightings Distribution web
page (Benson 2011).
Zebra mussels affect native clams and other species in a variety of ways. The reproductive cycle
of the zebra mussels differs from native clams in that there is no parasitic glochidia stage. The fer-
tilized eggs develop into larvae (the veliger), which loat in the water column (are planktonic) from
2 weeks to several months, they then settle, develop into juveniles and can become adults within a
year. Their high reproductive rate and rapid growth give them a competitive advantage (Morales-
Chaves 2004). Zebra mussels form thick encrustations on a variety of substrates, including clams

Freshwater mussels

69

Crayfishes

51

Stoneflies 43

Freshwater fishes

37

Amphibians

36

Flowering plants 33

Gymnosperms

24

Ferns/fern allies 22

Tiger beetles 19

Presumed/possibly extinct (GX/GH)
Critically imperiled (G1)
Imperiled (G2)
Vulnerable (G3)

Butterflies/skippers 19

Reptiles 18

Dragonflies/damselflies

18

Mammals 16

Birds

14

0% 10% 20% 30% 40%

Percent of species

50% 60% 70%

FIGURE 6.13 Proportion of species at risk by plant and animal group. (From Grabarkiewicz, J.D. and Davis,
S.W., An introduction to freshwater mussels as biological indicators, including accounts of interior basin,
Cumberlandian, and Atlantic slope species, U.S. Environmental Protection Agency, Washington, DC, 2008.)